1. Field of the Invention
The present invention relates to a controller and a control method for a motorised vehicle for example an electric wheelchair, wherein the controller receives control signals from a user input device.
2. Description of the Prior Art
Conventional electric wheelchairs have drive wheels which are driven independently by a motor arrangement. Whilst the motor arrangement used to drive the wheels may comprise a single motor with independent couplings between the motor and the wheels to effect independent drive of those wheels, more commonly each driven wheel is driven by an independent motor. Controlling the speed and direction of each driven wheel allows the user to manoeuvre the wheelchair.
A typical input control interface for an electric wheelchair provides a joystick having two control axes, one to control the forward/reverse speed and one to the control the turn rate (or spin). Although such control interfaces ostensibly provide the user with free control over the wheelchair's speed and spin, it is in fact common to impose certain limits on the combinations of speed and spin which the user can request. This is done essentially for safety reasons, because for example a maximum spin request (with a left wheel and a right wheel being driven in opposite directions) might be safely requested when the wheelchair is stationary (to thus pivot on the spot), but such a sharp turn could be dangerous if requested when the wheelchair is already travelling with a substantial linear speed. In a worst case scenario this could cause the wheelchair to topple over.
It is therefore known to impose “gate shaping” on the control interface, which typically allows a greater degree of spin to be requested in combination with lower linear speeds, whilst limiting more restrictively the spin that may be requested in combination with high linear speeds. Such gate shaping is typically implemented by means of an algorithm which transforms signals from the user's control interface into signals which control the motor arrangement of the wheelchair.
Even without entering a regime where the wheelchair is liable to tip over, management of the speed and spin of the wheelchair is still necessary because of the effect that certain combinations of speed and spin can have on the fraction of each wheel during a turn. A loss of traction of one wheel can have undesirable consequences for the handling of the wheelchair when making a turn. One reason why traction may be lost is the centripetal force acting on the wheelchair during a turn, which will tend to lift the inside wheel thus reducing its traction. This problem is worse with front wheel drive wheelchairs since the inside wheel is the high torque wheel and is thus more likely to lose traction. If a significant degree of traction is lost on the inside wheel, then the wheelchair may be liable to spin out of control into the turn. Even without such drastic lost of control, even a smaller loss of traction may cause handling difficulties for the user of the wheelchair. In general a loss of traction of either driving wheel would ideally always be avoided.
Conventional gate shaping approaches to this problem can certainly prevent the wheelchair from overturning, but nevertheless suffer from problems in the handling characteristics of the wheelchair due to the imposed limits on the relative speed and spin affecting the smooth steering feel of the wheelchair, and have led to the overly severe inhibiting of the spin speed in certain joystick demand regions.
An example set of the parameters that may be predetermined in a contemporary wheelchair controller are given in section 4.1 of the “DX2-REM550/REM551 Installation Manual”, March 2008, produced by Dynamic Controls.
Some further background technological information to the present invention can be found in the “enAble40 Powerchair Control System” manual produced by Curtis Instruments, Inc. of New York, USA; in U.S. Pat. Nos. 5,033,000 and 5,307,888; and in US Patent Application Publication 2010/0007299 A1.
It would be desirable to provide an improved technique for controlling motorised vehicles which would alleviate the above-discussed problems.